Eemu Ranta - Hydrothermal fluids

Sources of volatiles in volcanic hydrothermal systems

One of the fundamental scientific questions of hydrothermal research is to determine the source of volatiles present in geothermal gases and liquids. Different volatile species can have different origins, such as the mantle, magma bodies at varying depths, local groundwater (actually > 95% of the volume of Icelandic geothermal fluids), or leaching from rocks along hydrothermal fluid pathways. Superimposed on the primary chemistry of the fluids are the effects of secondary processes that take place between their source and the surface. The most important secondary processes are fluid mixing, fluid-rock interaction and phase transformations (boiling, condensation). The job of a hydrothermal geochemist is to untangle this web of secondary processes from measured chemistry and isotopic catalogue.

What are the sources of volatiles in volcanic hydrothermal systems? How important are these systems as volatile conduits from the mantle and the crust to the atmosphere? We tackle these basic questions at two relatively unexplored and remote geothermal areas of the Kverkfjöll and Askja volcanoes in central Iceland.

The first aim of the project is to characterize the geochemistry of the two geothermal systems and to build an understanding over how they work. The second aim is to use the geochemical dataset for identifying and quantifying the volatile sources that supply the systems.

Approach. Samples of fumarole gases as well as thermal and non-thermal waters were collected during four field campaigns between 2017-2019. We then analyzed the major element chemistry as well as the isotopic composition of hydrogen, oxygen (in H2O) and sulfur (in H2S or SO42-) in the samples. In addition, samples were collected for analyzing He, B and C isotope ratios as well as C and H isotopes in hydrocarbons. What comes in handy for the project is our collection of volatile concentration and isotope data from the host rocks of the geothermal system at Kverkfjöll, as these can be used to tune our geochemical models.

Sampling and analytical techniques

Sampling of natural geothermal waters and gases is a lot of fun and quite different compared to hard rock geology. Sampling sites are behind long hikes, and sampling itself is a slow, zen-like process that requires full concentration. Different types of samples need to be collected at the same site for different types of analyses, so on a good day, we may be able to collect four full samples. The chemistry of the samples is determined by gas chromatography, optical emission spectrometry, mass spectroscopy and titrations when back in the lab. Stable isotope ratios are measured with various extraction and spectrometric techniques depending on the element.

Top. Jói sampling a boiling temperature fumarole gas into a two-port glass flask at Víti, Askja. The flask is prepared in the laboratory before the field trip with a basic solution (KOH) to trap CO2 and H2S, and is evacuated to a vacuum. The vacuum is necessary to convince the fumarole gas to flow into the bottle. The bottle is kept cool by pouring cold water over it, which causes the water steam to condensate in the flask. Bottom. Home-built sample introduction system for the gas chromatograph, which we use for analyzing the concentrations of non-condensible major gases and hydrocarbons in geothermal gas samples. The sample is moved between diffent compartments of the loop with the help of digital valves.

Heating up surrounding groundwater is the easiest way for a shallow magmatic intrusion to cool itself. The result is a volcanic geothermal system, an underground convection cell that works as an interface between a magmatic intrusion and the atmosphere. Rising 1-phase fluids boil upon decompression. Fumaroles and acid-sulfate ponds represent the acidic gas portion of these fluids. As a rare natural example, the boiled liquid part can also be sampled at Kverkfjöll, where the alkaline-carbonate Hveragil river flows out the flank of the mountain. Standing in Hveragil, from left to right: Dan Colman, Jóhann Gunnarsson Robin, Andri Stefánsson.

Publications / presentations

Ranta E, Halldórsson SA, Barry PH, Ono S, Gunnarsson Robin J, Kleine BI, Ricci A, Fiebig J, Sveinbjörnsdóttir ÁE, Stefánsson A (2023) Deep magma degassing and volatile fluxes through volcanic hydrothermal systems: Insights from the Askja and Kverkfjöll volcanoes, Iceland. Journal of Volcanology and Geothermal Research 436:107776 (link)
Ranta E, Stefánsson A, Gunnarsson-Robin J, Kjartansdóttir R, Halldórsson SA. Characterising Geothermal Fluids of the Askja and Kverkfjöll Volcanic Systems, Iceland. World Geothermal Congress 2020+1, Reykjavik, Iceland (October 2021). Oral.
Ranta E, Stefánsson A, Gunnarsson-Robin J, Kjartansdóttir R, Halldórsson SA (2021) Characterising Geothermal Fluids of the Askja and Kverkfjöll Volcanic Systems, Iceland. Proceedings World Geothermal Congress 2020+1 (extended abstract)
Ranta E, Stefánsson A, Gunnarsson-Robin J, Kjartansdóttir R, Halldórsson SA, Barry PH, Ono S. (2020) Geochemistry of the hydrothermal systems of the Askja and Kverkfjöll volcanoes, Iceland. Nordic Geological Winter Meeting 2020, Oslo, Norway. Poster.